In a contemporary hard disc drive, a pivot bearing assembly provides for rotatably supporting a head stack assembly within a head disc assembly so that each transducer head incorporated within the head stack assembly can be swung into a desired position relative to a respective recording surface of a disc. The head stack assembly typically includes an actuator body portion surrounding a bore opening, a voice coil motor portion, and a set of head gimbal assemblies each carrying at least one transducer head for reading and writing on the recording surface. The actuator body portion of the head stack assembly is typically attached to the pivot bearing assembly by inserting the pivot bearing assembly into the bore of the actuator body portion.
A conventional pivot bearing assembly comprises a shaft that is fixed to the base of the enclosure for the head disc assembly. The conventional pivot bearing assembly also includes at least two sets of bearings, and an outer sleeve. Each set of bearings generally has an inner race, an outer race, and ball bearings between the inner and outer races. An inner surface of each inner race abuts a portion of an outer cylindrical surface of the shaft. An outer surface of each outer race abuts a portion of an inner cylindrical surface of the outer sleeve. As installed within the head disc assembly, the outer surface of the cylindrical sleeve abuts the interior cylindrical surface of the bore of the actuator body. As a result, the actuator body portion is able to rotate about the fixed shaft of the pivot bearing assembly.
A contemporary disc drive needs to meet exacting standards with respect to the speed with which data can be accessed. These exacting standards involve high speed seek operations during which the head stack assembly receives high torque for angular acceleration to depart from a starting track and ramp up to a high angular velocity, then receives high torque for angular deceleration to ramp down the angular velocity and bring the active transducer head to a stable position at a target track for track following.
During such high speed seek operations, various structures involved in supporting the transducer heads can vibrate in a manner characterized by at least one resonant frequency. So long as the active transducer head is vibrating by a sufficient amount after the deceleration torque is removed, the drive cannot begin writing to or reading from the target track. Also, the vibrations propagate from the head stack assembly through the pivot bearing assembly and into a base plate and cover causing increased acoustical noise.
One source of such vibrations involves the ball bearings within the pivot bearing assembly. The ball bearings are characterized as having a finite radial stiffness. The existence of such finite radial stiffness, and particularly the tolerances associated with it (both initial tolerances and variations resulting from wear of the bearings), pose problems in disc drive designs.
A traditional approach to damping vibrations that occur in a pivot bearing assembly is to use O-rings that are compressed between portions of the pivot bearing assembly. For example, U.S. Pat. No. 5,727,882 describes a prior art approach of compressing a first O-ring between a cap portion and an inner sleeve and a second O-ring between the inner sleeve and the fixed shaft of the pivot bearing assembly. U.S. Pat. No. 5,666,242 discloses similar elastomeric interfaces.
A disadvantage of such a technique is that the damping provided by the O-rings is tolerance dependent and hence is not consistent among a common lot of O-rings. Therefore, when slight manufacturing differences exist among a common lot of O-rings, the damping provided by the O-rings changes.
U.S. Pat. No. 5,727,882 discloses alternative techniques for damping vibrations in pivot bearing assemblies. In particular, a channel is formed in the sleeve that surrounds the shaft. The channel contains a cured cast-in-place material that dampens vibratory waves propagating between of the sleeve and the shaft. Alternatively, the channel can be located between the shaft and the set of bearings. The channel is formed by cutting the sleeve. In another embodiment shown in FIG. 4 of the '882 patent, the shaft has ring shaped channels and the cured material must be post processed, i.e., trimmed of excess material.
Such a technique, however, has the disadvantage of requiring additional manufacturing steps such as creating the channel and curing the cast-in-place material.
U.S. Pat. No. 5,675,456 discloses an acoustic vibration decoupler for decoupling acoustic actuator vibrations in an actuator pivot bearing assembly. The vibration isolator includes an inner and outer member coupled by a plurality of pliable elements. The isolator is preferably fabricated as a single injection molded piece. U.S. Pat. No. 5,491,598 discloses a rotary actuator vibration damper. Damping material is coupled to the inner surface of the sleeve and an auxiliary member is coupled to the opposite surface of the damping material.
These techniques suffer from several disadvantages. First they typically require more manufacturing and/or assembly steps than the assembly of a pivot bearing without any damping capabilities. These additional steps increase the time needed to manufacture and/or assemble the pivot bearing assembly with such damping capabilities. In addition, the increase in manufacturing and/or assembly time results in an increase in the cost of pivot bearing assembly with such damping.
Therefore, there is a need for a cost effective and simple vibration damping technique that will adequately isolate vibrations without increasing the time and cost of manufacturing and assembling a pivot bearing assembly with such vibration isolation.
The present invention provides a solution to the above and other problems and offers the above and other advantages over the prior art.